The term magnetometry refers to the detection and measurement of magnetic fields produced by biological and non-biological samples. The term biomagnetometry refers to a subclass of magnetometry that applies to the detection and measurement of magnetic field produced by biologic organisms and samples of tissue taken from such organisms. One specialty within the general field of biomagnetometry is magnetoencephalography (often abbreviated by the acronym “MEG”). MEG refers to the detection and measurement of the magnetic fields which are produced by the electric currents which flow naturally within the brain of humans and animals. Charged ionic flows within the neurons which make up part of the human brain are, in effect, an electric current which produces a magnetic field which can be measured using the methods of MEG. The electric currents which drive the pumping of the heart in an animal produce magnetic fields which can be measured using magnetocardiography (often abbreviated by the acronym “MCG”), another specialty within biomagnetometry. Measurements of the magnetic fields produced by these electric currents can be used to deduce information about the size and direction of the currents as a function of time as well as their location and distribution within the body of a person, and therefore to provide information about the state of health and the state of function of the person.
Apparatus and methods of MEG and MCG have been developed and expanded over the past forty years, enhancing sensitivity to enable the detection of magnetic fields produced by electric currents flowing deep within the body. The “field of view” of the biomagnetometers has been systematically expanded from single channel detectors of the magnetic field at one location to large helmet-shaped systems measuring the values of the magnetic fields at up to 275 locations around the head of a human being or up to 150 locations over the chest of a human being.
Biomagnetometry has also been used to measure magnetic fields produced by electric currents flowing in biologic samples such as brain tissue slices of laboratory animals. In these systems, methods have been developed to bring the detector of the magnetic field as close as possible to the electric current itself to maximize the size of the measured signal and the ratio of the signal to the background magnetic noise. In some case, spacing as small as 1 mm or less have been achieved. This close spacing also enables enhanced spatial resolution—the ability to more precisely determine the spatial distribution of the electrical currents in the biologic sample which are producing the measured magnetic field.
Generally, the biomagnetic measurements of biogenic electric currents are useful for measuring the distribution of such currents in a tissue such as a brain slice or in an organ such as a brain or heart. However, the magnetometry using the same techniques as in biomagnetometry can be applied to measuring the electric currents flowing in non-biological objects. For example, non-destructive testing of electronic circuits can be performed by determining the electric current flow patterns in those circuits during their operation by measuring the magnetic fields produced by those currents. Because magnetometry can have very high levels of frequency response, subtle and otherwise undetectable malfunctions during high frequency operation may be identified in this manner. In non-destructive evaluation of materials such as a portion of the jet wing, for example, the magnetometry can be used to detect the presence of cracks by measuring the magnetic field associated with induced electric currents in the material.
The use of magnetic measurement techniques to provide information on the spatial distribution of electrical currents with very high precision can currently be implemented in scanning devices such as scanning magnetic microscopes. Such scanners use methods for moving either the sample being scanned or the magnetic measurement device in a very precise and well calibrated manner to collect the spatial distribution information.
However, a major limitation in the application of the biomagnetic techniques for the purposes outlined above arises from a fundamental property of magnetic fields produced by electric currents flowing in tissues or organs. Any electrogenic biological tissue or organ can be described as a circuit of electrically active cells that produce the biogenic current. In intact humans or animals the tissue of the organ that contains such electrogenic cells is saturated with physiological saline. In in vitro preparations, such a tissue is immersed in a bath of physiological saline. The physiological saline conducts electricity; thus, the medium containing the saline such as the brain or the head, or a bath containing the tissue, is called a “conductive medium.” (The term “conductive” used here and throughout this disclosure, including the specification, drawings, claims, and abstract, refers to the property of electrical conductivity and should be interpreted as such unless otherwise stated.) A homogeneous conductive medium is a conductive medium in which the electrical conductivity is uniform throughout the conductor as is the case for a saline bath containing a biological tissue. A non-homogeneous or heterogeneous conductive medium is a medium in which the electrical conductivity is non-uniform. The brain and heart are, strictly speaking, non-homogeneous media, but they can each be very well approximated as a homogeneous medium.
From the fundamental principles governing electromagnetism in homogeneous conductive media, an electric current which flows within and proximate to the surface of such a conductive medium and flows in a direction which is perpendicular to the surface of that medium produces no net magnetic field external to the medium itself. This is strictly true when the conducting medium is spherical or flat. A large bath can be thought of as a part of an infinitely large sphere. But it is very well approximated even in a conducting medium that lacks a spherical symmetry when the cells are close to the boundary separating the conducting medium from the surrounding non-conducting medium. This factor has limited the utility of biomagnetic measurements such as MEG in providing complete information about electric currents in a tissue or in an organ. Conventional biomagnetic techniques can provide the information only about those components of the electric currents flowing within homogeneous conducting media which flow in a direction parallel to the surface of that medium, but not the currents which flow normal to the surface. In other words, conventional biomagnetometry provides the information about one component of the biogenic current but not the other, thus providing incomplete information regarding the total current produced by biological generators of electric currents. In general, this factor has significantly constrained the application of the biomagnetic techniques for measuring biogenic currents from various organs including the brain and heart in humans and animals.
Similar limitations apply for measurements of some non-biological samples which present current flows in a homogeneous conductive medium. The same limitations also apply to scanning magnetometers constructed to provide high spatial resolution measurements of the distribution of electrical currents in biological and non-biological materials and samples. If this limitation is removed, the utility of such scanning magnetometers would be expanded.
Apparatus and methods for removing this limitation in the field of biomagnetic measurement has been recently developed and has been disclosed in U.S. patent application Ser. No. 13/040,027, filed 3 Mar. 2011. This apparatus requires the placement of a non-conducting object known as a primary source mirror (hereinafter PRISM), within a conducting medium in close proximity to the electric currents of interest. Apparatus and methods for expanding the basic method of a single primary source mirror by employing large numbers of such mirrors concurrently and by packaging arrays of PRISMs into modules has been disclosed in U.S. patent application Ser. No. 13/206,392, filed 9 Aug. 2011. The methods disclosed in these applications can be configured and applied in a novel manner to create a scanning magnetometer which does not suffer from the limitations described above concerning the incomplete measurements of biogenic and non-biogenic currents.
The subject matter disclosed in the Ser. No. 13/040,027 application enables a complete characterization of the generator of electric current by introducing a device called a PRISM that effectively transforms a homogeneous conductive medium into a non-homogeneous conductive medium. In both biological and non-biological conductive media, there are some cases in which the conductivity in the medium varies sufficiently so that the biogenic current perpendicular to the overlying conductivity boundary (such as the scalp-air, chest-air boundary, or saline-air boundary) can, in principle, produce a non-zero magnetic field outside the conductive medium. However, in all such cases the precise geometry and nature of the heterogeneous property of the medium is not known since it is determined physiologically or physically. Therefore it is not possible to use this feature of naturally occurring non-homogeneity to accurately measure the otherwise nondetectable perpendicular current generators. The PRISM device of the '027 application provides a means to selectively measure the magnetic field from the electric current generator of the magnetic field in a target region of the sample without interference from the current generators in other regions of the sample. The PRISM invention furthermore provides a means to increase the spatial resolution of the target volume by at least one order of magnitude compared to conventional magnetometry techniques since the spatial resolution is determined by the distance between the PRISM and the sample, not by the distance between the sample and magnetic field detector and by the size of the magnetic field detector as is the case in conventional magnetometry. This PRISM invention, however, does not provide a means to determine the distribution of currents in the entire sample. The PRISM array in the '392 application provides a means to determine the current distribution over the entire sample, but the spatial resolution is determined by the size and spacing of the PRISMs in the array, which will be relatively coarse in practical applications.
The scanning magnetometry invention embodiments described here extend the utility of the above two inventions by enabling the determination of the spatial distribution of currents in a sample with high-resolution without being limited by the size and spacing of PRISMs in an array or by the size of the magnetic field sensors. This invention then significantly expands the capability of scanning magnetometers to measure the location, magnitude, and direction of electric currents flowing in a conductive medium.